The present invention relates to an electronic driving circuit for a selective use of two electro-acoustic transducers of a mobile terminal.
Common mobile terminals like e.g. mobile phones, personal digital assistants (PDA) or the like, are equipped with an earpiece speaker and a buzzer as standard acoustic components. The earpiece speaker is hereby typically used to transduce an audio signal provided by a voiceband electronic circuit of the mobile terminal into a respective sound signal. Other signals, like e.g. ringing tones or alarms are usually produced in form of square wave signals with variable pulse width and are transduced into a respective sound signal by a so-called buzzer. On many current mobile terminals, particularly on high-grade models, the buzzer is replaced by a hands-free speaker, which will in the following be referred to as loudspeaker. This loudspeaker usually not only transduces square wave buzzer signals, but is also used for reproducing ringer tones which are often provided in the form of polyphonic melodies. The loudspeaker is further used to transduce audio signals originating from the voiceband electronic circuit like the earpiece speaker but at a higher sound level than that, for to enable a user to operate the mobile terminal in hands-free mode. For this purpose, an audio power amplifier external to the voiceband electronic circuit is wired between an output of the voiceband electronic circuit and the loudspeaker. The loudspeaker is either connected to a separate output of the voiceband electronic circuit or it shares the only one output provided with the earpiece speaker.
The outputs provided by the voiceband electronic circuit are either single ended or balanced. The voiceband electronic circuit is typically formed by an integrated circuit which allows to configure the details of its last processing stage supplying an audio signal to the one or more output ports of the voiceband electronic circuit. The audio signal on one output port may thereby be of equal or different phase than at another port of the outputs provided. The audio signals present at the one or more outputs are then processed by an external circuit which supplies the two speakers.
Three different types of external circuits are currently used to wire the speakers, i.e. the loudspeaker and the earpiece speaker to a voiceband electronic circuit. In a first type, each speaker is supplied from a separate, single-ended or balanced output of the voiceband electronic circuit. A balanced output provides the signal at each of its two ports. By default the phase of a signal at a first port is of opposite phase to that of the respective other port. Different from the general meaning, the term ‘balanced output’ as used in this specification denotes an output where the phase of a signal at a first of its ports has a defined relationship to that of the signal present at the respective other port.
A second commonly used external circuitry uses just one output of the voiceband electronic circuit for supplying both, the earpiece speaker and the loudspeaker. While the earpiece speaker is usually directly connected with the output, a power amplifier is set between the output and the loudspeaker for additionally amplifying the audio signal. The audio power amplifier typically comprises a control port by which the amplifier can be switched on or off. There are only two modes of operation possible. In the first, only the earpiece speaker is active, in the second both, the earpiece speaker and the loudspeaker are active.
Very often, the last type of external circuit is modified to such as to additionally allow a square wave buzzer signals to be transduced by the loudspeaker. The audio power amplifier for the loudspeaker is wired in this third type of external circuit for to form an adder circuit which allows the loudspeaker to reproduce the audio signal as well as the buzzer. In this commonly used type of wiring, an additional analogue switch is necessary to disconnect the audio signal from the audio power amplifier when transmitting the buzzer signal. Otherwise the audio signal from the voiceband electronic circuit output would be audible when the audio power amplifier has to be turned on for reproducing the buzzer signal.
Although, the last two types of external circuitry are usually preferred as only one output for the audio signal has to be provided at the voiceband integrated circuit, these types of wiring are disadvantageous in that the audio signal is always present on the earpiece speaker of the respective mobile terminal.